At present, waste water or sewage is purified essentially by a method consisting in causing a culture of bacteria dispersed in treatment basins to develop in the presence of oxygen (with such a culture also being known as a free culture or activated sludge), then, after sufficient contact, the purified water is separated from the sludge by sedimentation in a clarifier, with a fraction of the sludge being recirculated to one of the treatment basins in order to maintain a sufficient concentration of purifying bacteria therein while the remainder of the sludge, representing excess activated sludge, is removed from the installation. Such a purification method seeks both to eliminate organic carbon pollution and to oxidize nitrogen pollution by nitrification. By including periods of contact between the activated sludge and the water to be treated in the absence of aeration (anoxic contact) and in the presence of carbon, heterotrophic microorganisms are caused to degrade nitrates into gaseous nitrogen in order to perform denitrification. In addition, by subjecting the microorganisms to systemic alternation between anaerobic and aerobic conditions, it is possible to cause phosphor-containing compounds to accumulate in excess in the microorganisms, thereby giving rise to biological dephosphatization of the water being treated. Over the last 20 years,numerous variants of that method including various possible dispositions of anaerobic, anoxic, and aerobic zones have been developed and have given rise to numerous patents and publications. Unfortunately, it has been observed that in methods using activated sludge, the parameter limiting nitrification is not the reaction kinetics of transforming ammonia or organic nitrogen into nitrate, but the age (or real retention time) of the sludge which is necessary for conserving nitrifying microorganisms in the installation. As is known, the times required for forming such nitrifying microorganisms are very long and vary strongly with temperature: thus, at a temperature of 11.degree. C., the sludge must be 10 days old, and it may be assumed that for temperatures below 11.degree. C., the growth of such microorganisms is slowed down very considerably. In addition, these microorganisms which are obligate aerobes are capable of growing in an aerated zone only, so the retention time that needs to be taken into consideration is the time spent in the aeration basin. As a result, it is necessary to provide aeration basins of large volume that are overdimensioned relative to reaction rate.
If the age of the sludge is written A, the temperature of the biological basin in .degree.C. is written T, and the daily production of excess sludge is written Px kg of solids/day!, then the mass of activated sludge, being aerated for nitrification purposes, Mba, is given by the following equation: EQU Mba=A*Px (1)
with: EQU A=4.5*0.914 .sup.(T-20) ( 2)
which, according to the proposals of the German organization Abwasser Technischen Vereinigung E. V. (abbreviated below as ATV) gives rise to a daily sludge production Px of EQU Px=0.6*(MES+BOD.sub.5)-(0.072*0.6*K)/(1+0.08*K)!*BOD.sub.5( 3)
where:
MES=daily throughput of solids admitted to biological treatment (in kg/day); PA1 BOD.sub.5 =daily throughput of 5-day biological oxygen demand admitted for biological treatment (in kg/day) with: EQU K=A*1.072 .sup.(T-15) ( 4) PA1 V=aeration volume (m.sup.3) PA1 Mba=mass of activated sludge (kg) PA1 Cba=concentration of activated sludge (kg/m.sup.3). PA1 Tps=thickening time of sludge being clarified (hours) PA1 ISV=sludge volume index (ml/g) PA1 K is a constant equal to 0.6 or 0.7 depending on the sludge takeup technique. PA1 a) The high concentration of substrate in the superactivation reactor makes it possible, because of the exothermal nature of the oxidizing reactions, to obtain temperatures (25.degree. C. to 40.degree. C.) in the reactor which are highly favorable to the development of microorganisms in general and in particular to the development of the nitrification microorganisms. Thus, if equation (2) is applied, the sludge age that needs theoretically to be maintained in the reactor in order to obtain development of the nitrification microorganisms lies in the range 2.8 days to 1 day for temperatures lying in the range 25.degree. C. to 35.degree. C. PA1 b) By keeping the biological culture permanently in strict aerobiosis in the superactivation reactor, it is possible to obtain optimum conditions for growth of the strict aerobic microorganisms (such as the autotrophic microorganisms that perform nitrification), and to exert selection on microorganisms that are obligate anaerobes, or optional anaerobes. In addition, the power of the biological reactions implemented gives rise to considerable degradation of the substrate (about 35% to 50% of organic matter is eliminated) and thus of polluting particles, thereby reducing sludge production by 20% to 30%. PA1 a mere step of oxidizing the carbon pollution, performed in a biological reactor operating under aerobic conditions; PA1 a step of oxidizing the carbon pollution plus a step of nitrification which take place under aerobic conditions and generally in the same biological reactor; PA1 a step of oxidizing the carbon pollution, a step of nitrification, and a step of denitrification which can be performed in the same biological reactor, the first two steps taking place under aerobic conditions while the last step takes place under anoxic conditions, the reactor operating alternately in aerobiosis and in anoxia; PA1 a step of oxidizing the carbon pollution, a step of nitrification, and a step of denitrification, the denitrification step taking place in anoxia in a biological reactor that is distinct from the reactor for oxidizing and for nitrification, and that is situated upstream therefrom and through which a fraction of the contents of the oxidizing and nitrification reactor is recycled; and PA1 a step of oxidizing the carbon pollution, a nitrification step, a denitrification step, and a dephosphating step, the dephosphating step taking place in an anaerobic biological reactor which is situated upstream from the denitrification reactor and in which there is recycled the waste water separated from the activated sludge which is collected at the end of the system.
The dimensions of the aeration volume are then given by the following equation: EQU V=Mba/Cba (5)
where:
Several solutions have been proposed for reducing the aeration volume.
For example, it has been proposed to increase the concentration (Cba) of the biomass being aerated. If Qt is the through flow rate, Qr the recirculation flow rate, and Cr the concentration of the substance being recirculated, then the mass equilibrium equation for the sedimentation clarifier is as follows: EQU Qr*Cr=(Qt+Qr)*Cba (6)
from which it can be deduced: EQU Cba=(Cr*Cr)/(Qt+Qr) (7)
Since Cr is necessarily greater than Cba, the clarifier must therefore perform a thickening function to enable the concentration Cr to be achieved. However, the thickening concentration of a sludge is a function of thickening time (Tps) and of the mechanical characteristics of the sludge. Such mechanical characteristics can be expressed by the volume sedimentation index in ml/g (i.e. the volume occupied by 1 gram of sludge after 30 minutes of sedimentation), which mechanical characteristics are written below as ISV. As proposed by ATV, it is possible to write: EQU Tps=(Cr*ISV)/(1000*K)!.sup.3 ( 8)
or: EQU Cr=K*(1000/ISV)*Tps .sup.1/3 ( 9)
with:
Unfortunately, since the thickening of sludge in clarification takes place in the absence of added oxygen, if it is desired to avoid degrading treatment performance and the mechanical quality of the sludge then Tps must necessarily be less than, or at most equal to, the time the actived sludge spends in anaerobiosis. A Tps of 3 h is generally considered as being the maximum value. Consequently, in the presence of a very good ISV corresponding to 100 ml/g, the recirculation Cr cannot exceed 10 g/l if equation (9) is applied.
In addition, Qr is limited firstly by the volume of the clarifier allocated to thickening sludge, and secondly by problems of distributing energy at the inlet of the clarifier. For both of those reasons it is common to adopt a recycling ratio (where recycling ratio is defined as Qr/Qt) equal to 100% or rarely 150% of the raw water flow rate. Using the preceding example of an ISV of 100 ml/g and according to equation (7), the recycling ratio gives rise to a concentration of biomass in aeration Cba lying in the range 5 kg/m.sup.3 to 6 kg/m.sup.3, which is the concentration conventionally observed in water works.
Another means of increasing the concentration of the biomass in aeration is to use a system comprising firstly a "contact" basin where nitrogen in the form of ammonia is transformed into nitrates (nitrification) and secondly, on the sludge return circuit, a "stabilization" basin which enables a high value of sludge age to be obtained. Under such circumstances, the above-described constraints are found again, i.e. activated sludge concentration is equal to Cba in the contact basin and the concentration in the stabilization basin is equal to Cr, which represents only a small overall reduction in the volume of the works.
EP-A-0 309 352 describes a method in which obtaining the desired age for the sludge is performed in a generation basin located in a loop outside the reaction basins and wherein, prior to recycling the sludge through at least one purification process, a step of concentrating said sludge by floatation is performed upstream from the generation basin. That method makes it possible to achieve a considerable reduction in the volume of the Works in that the problem of obtaining aged sludge is treated in a basin that operates at high concentration (30 g/l) so the reaction basins can then be dimensioned as a function of reaction rates.
The essential problem in operating conventional activated sludge methods is that it is essential to obtain sludge having at least a minimum age while seeking high reaction rates, and this must be done in a reaction context which is favorable to neither of those two parameters. Studies have shown that those parameters and objectives are partially incompatible because the specific activity of sludge decreases with its age.